Submarines don't turn CO2 back into O2. They "scrub" the CO2 out using Soda Lime (or similar). They use electrolysis or oxygen candles to generate new oxygen.

https://www.analoxgroup.com/blog/how-do-you-breath-onboard-s...

After the soda lime is maxed out, it's tossed. At least so far as I understand you can't meaningfully re-use it for scrubbing, though there are other industrial uses where it can be re-purposed. Also O2 electrolysis takes a fairly substantial amount of electricity, which is OK when you've got a nuclear generator on your hands.

So.... it's a lot cheaper (and more environmentally friendly) to use an ERV system in your HVAC system even at "only" 80% recovery. Let the plants do what they are good at and deal with capturing CO2 and giving us O2.

The problem is we don't generally build insulated enough or tight enough houses to matter. If everything was built to passive house standard, we'd save HUGE amounts of energy globally.